Jenkins, P. J.
(After stating the foregoing facts.) 1. Where one enters into a contract with a manufacturer to install for him a sprinkler system in his store, and agrees therein to pay a stipulated price, but, instead of permitting the installation, notifies the manufacturer that he will not permit it, the manufacturer, upon proof of readiness, willingness, and ability to perform the contract, is entitled to damages for breach of contract. The measure of the damages is the difference between the contract price and what it would have cqst to perform the manufacturer’s part of the contract. White & Hamilton Lumber Co. v. Lynch, 159 Ga. 283 (125 S. E. 472); Campbell v. Mion, 6 Ga. App. 134 (3) (64 S. E. 571); Wallace v. Tumlin, 42 Ga. 462 (4), 463; 8 R. C. L. 511. To this amount it is,within the discretion of the jury to add legal interest from the time of the breach to the time of the recovery (Civil Code (1910), § 4396); but it is error for the court to direct that such interest shall be added by the jury, the matter being wholly within their discretion. Snowden v. Waterman, 110 Ga. 99 (35 S. E. 309); Patterson v. Peterson, 15 Ga. App. 680, 684 (84 S. E. 163).
2. Where the manufacturer sues for damages for the breach *10of a contract as set forth above, in order to recover it is necessary to prove (1) the contract price, and (2) what it would have cost the manufacturer to perform the' contract. In order to prove such cost there must be testimony as to what materials and labor were necessary, and as to the cost or value of the materials and labor necessary at the time of the breach, and not at the time of the original making of the contract. The estimated value of the materials and cost of labor necessary to perform the contract, reckoned as of the date of the contract, is not the proof necessary in measuring the damage; their value and cost at the time of the breach fixes the measure. It may be that there was either an increase or a decline of such value and cost between the time the contract was made and the time it was breached; either of which would materially affect the measure of damages. If there was any fixing of the value or cost of materials and 'labor in this ease, it was fixed as of the date of the making of the contract, and not as of the date of its breach; and there was no testimony that the cost or value remained unaltered during that interim. Therefore the evidence was not sufficient to sustain the verdict.
3. In such a suit it is necessary for the plaintiff, in. order to make out a liability for actual damages, to show the value of the materials and labor necessary to the plaintiff’s performance of the contract. Civil Code (1910), § 5875. The jury are not bound by the opinion of experts as to value. Atlantic & Birmingham Ry. Co. v. Howard Supply Co., 125 Ga. 478 (2 b) (54 S. E. 530); Martin v. Martin, 135 Ga. 162 (68 S. E. 1095); Wilson v. Baltimore, 135 Ga. 469 (2) (69 S. E. 740). The credibility of testimony is for the jury. Clary v. State, 8 Ga. App. 92 (2) (68 S. E. 615). Accordingly, in this case, where there was evidence (as set forth in the statement of facts) which, properly construed, went only to the extent of proving the contract price of a long-list of materials, labor, etc., but no evidence of any conventional agreement between the contracting 'parties as to the value and cost of the various items of the contract, as in cases relating to the face value of notes, bonds, collaterals, etc., and where there was no evidence that the complaining party, as the quasi agent of the breaching party, went into the open market, after notice to the breaching party, and liquidated the value or cost, by actual sale or purchase, to cure such breach, there was no testimony in the
*11record, such as would indisputably show the value and cost of the materials and labor necessary to performance. It is only where the value has been in some way liquidated that proof of value and cost becomes that of substantive facts and ceases to be opinion testimony. So that in eases where there has been no liquidation of values as in cases referred to above, but their determination rests purely upon opinion testimony, as in the case at bar, the liquidation of values must be arrived at by a jury, and not by the court. See, in this connection, Baker v. Richmond City Mill Works, 105 Ga. 225 (31 S. E. 426); Finleyson v. International Harvester Co., 138 Ga. 247 (2) (75 S. E. 103); Georgia Northern Ry. Co. v. Battle, 22 Ga. App. 665 (97 S. E. 94); Birmingham Paper Co. v. Holder, 24 Ga. App. 630 (5), 631 (101 S. E. 692); Civil Code (1910), § 5875; Carter v. American Slicing Machine Co., 23 Ga. App. 422 (2), 423 (98 S. E. 365); cf. Lott v. Banks, 21 Ga. App. 246 (4), 248 (94 S. E. 322). The case of McNamara v. Georgia Cotton Co., 10 Ga. App. 669 (3) (73 S. E. 1092), cited by the defendant in error, is not in conflict with what is here held. Here the witness did not pretend to have expert knowledge or to testify as to what was the market value of the labor and materials required for the improvement. He testified only, in effect, as to what in his preliminary estimate made at the time of the contract the plaintiff would have to pay for the materials and what the witness then thought would likely be the cost of the labor required. In the McNamara case, supra, the witness qualified as an expert, showing that he was familiar with the market value of the fluctuating commodity, which had a fixed and recognized market value, and testified as to the fixed market value as at the time of the breach. See, further, in this connection: Bryan v. Southwestern Railroad Co., 41 Ga. 71; Southwestern Railroad Co. v. Rowan, 43 Ga. 412, 414; Atlantic Coast Line R. Co. v. Harris, 1 Ga. App. 667, 669 (57 S. E. 1030); Griffin v. State, 15 Ga. App. 520, 530 (83 S. E. 891). It was error for the court to direct a verdict for the plaintiff on the evidence appearing in the record.
Judgment reversed.
Stephens and Bell, JJ., concur.